Add enable_null_safety to core

[srawlins]

This rule: https://dart-lang.github.io/linter/lints/enable_null_safety.html

I propose we add it to a lints release that supports Dart 2.x, so that users can see if they have any pre-null safety files they forgot about, before using Dart 3.0 (like tests, tools, etc).

[srawlins]

CC @bwilkerson @pq as I think this is the only diagnostic which can report a // @dart=2.9 language version override in Dart 2.19.

[pq]

SGTM.

/fyi @natebosch @jakemac53 @mit-mit @lrhn @goderbauer @munificent

[lrhn]

If we make the lints package require SDK 2.19, it should not unduly affect code that still uses older SDKs.
Would we want that?

[srawlins]

I don't see anything wrong with keeping the minimum SDK version at 2.17.0.

[devoncarew]

FYI that we've since increased the min. required sdk here to 3.0, so if we do want this to resolve into a pre-3.0 sdk, we'd need to publish a release off the commit for package:lints v2.0.1.

[natebosch]

I don't think it's worth a backpublish for this. I think we should close this.

[devoncarew]

From a discussion, this would have been useful to add a few SDK releases ago, but will add little to no value at this point. Closing.